The increasing rise in the popularity of portable and mobile electronic devices, which are often powered by rechargeable batteries, has resulted in a corresponding need for efficient, low cost battery chargers that can recharge the battery (or batteries) in a reasonable period of time. All battery chargers that are powered by standard alternative current (AC) service perform an AC to DC (direct current) conversion. For low power chargers that provide a “slow” charge, the AC to DC conversion can be performed by simple rectification and current restriction. However, for “fast” charging, where a fully discharged battery can be fully charged in a matter of a couple hours, or less, the charging current and voltage requires more careful regulation, and typically employ switched mode power conversion techniques.
In some conventional rapid charging systems it is common to use an AC to DC converter to generate a “stepped down” DC voltage, which takes the input AC, rectifies it to a high DC level on the primary side, and converts the high DC to the stepped down DC voltage level on the secondary side. A DC to DC regulator, such as a “buck” regulator, is used to provide a regulated charging current and voltage to a battery from the stepped down DC level. This AC to DC to DC approach can be referred to as a “dual stage conversion” approach because the voltage is converted twice; from the input AC to the stepped down DC, then from the stepped down DC to the battery voltage.
The dual stage conversion approach works well, and allows the output to be varied over a wide range of voltage and current. However, because of the large number of power components involved, battery chargers using this kind of power conversion tend to be more expensive than a battery charger using a single conversion approach.
A single conversion power supply converts the input AC directly to the desired output charging current and voltage. Accordingly, there is no need for the DC to DC second stage. In a single conversion power supply control information can be communicated from the secondary side to the primary side over isolation links, such as opto-isolators. The isolation of the secondary, output side from the primary, input side of the power converted is necessitated by safety considerations to prevent the high AC from having a path to the secondary side, and is required by regulation in switched mode converters. The information that can be provided to the primary side can be used, for example, to control the electric current output, voltage limiting, and so on. For safety considerations it is desirable to sense output conditions on the primary side, such as by using an additional winding on the primary side. This is because of the time delay in sensing on the secondary side and communicating feedback across the isolation gap to the primary side control circuitry. The additional winding can be used to sense output voltage on the secondary side, and therefore allow for avoiding over-voltage conditions at the output.
Some chargers are required to charge different types of batteries that have different voltage requirements. What can be a typical voltage for one type of battery can be an unsafe, over-voltage condition for another type of battery. In a battery charger using a single conversion system, the sensing range produced by the additional winding may be too wide to sense an abnormally high voltage applied to a lower voltage battery. Typical charging limits are approximately two percent above 4.2V.
Accordingly, there is a need for battery charger using a single conversion power converter that is capable of charging multiple types of batteries having different voltage requirements while preventing over-voltage conditions when charging each type of battery.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.